Marconi TF-2700 Universal Bridge

Description

This is a battery powered one hand holdable universal LCR
bridge.
It operates at 1 kHz.
There is a provision for External AC drive as well as external DC bias,
which is why I got this instrument. So that I can make some
measurements
on the PNI Magneto-Inductive
(MI) sensors.

Power

The only way to power the bridge is by means of a 9 Volt
battery,
not the normal type but a larger Eveready 276.

DC Bias

The ground side of the 1/4" mono phone jack for "Bias" is
connected
directly to the "Hi" DUT terminal.
The center of the 1/4" jack is connected to the Det + ve" terminal
through a 1,000 Ohm resistor. The limit on bias current
translates
into a voltage limit at the jack.

DC bias is only active when in the L, C or R function positions
and
in the D mode and no change with position.

PNI SEN-L Data

With no bias applied the inductance looks like 102 mH and there is no
change
in inductance with magnetic bearing.
With 15 Volts (15 mA) DC bias the inductance reduces to 100 mH and
changes about 1 mH with position.
Using the R int DC function switch setting results in a DE resistance
reading of 120 Ohms.
Using the R function switch position ( 1 kHz gives 116 Ohms.

With 1.0 Volt DC bias the inductance is about 102.2 mH. D =
5.2
(X1 D range) This is closer to where the sensor should be biased.
With the coil pointing up the inductance is about 104 mH.

The normal frequency of operation for this coil is closer to 59 k
Hz,
so the 1 kHz TF-2700 frequency may be too low for good sensitivity
.
Using the Ext AC input may allow much more sensitivity. How to
use it?

Ranges

L

C

R

10 uH

1000 uF

1

100 uH

100 uF

10

1 mH

10 uF

100

10 mH

1 uF

1000

100 mH

0.1 uF

10 k

1 H

10,000 pF

100 k

10 H

1000 pF

1 M

100 H

100 pF

10 M

Construction

Discrete components and point to point wiring plus a single
sided PCB.